Geforce 9800gt SLI

I'am still very wet behind the ears when it comes to this stuff, But recently i bought a new pc and with it came 2 * geforce 9800gt 512 gc shiny I thought but has given me nothing but problems, I play cod4 eve and supreme comander mainly.

Problems i'am getting are when playing cod4 i can play for about 10 mins then i get a load of weird graphic errors ( screen going a nice rainbow color) and i'am unable to do anything other than do a force reboot, I have got the latest drivers from nvidia,, Disabled sli , and a few other tweeks but to no avail now its started doing it on my other games as well